U.S. patent number 5,770,887 [Application Number 08/320,263] was granted by the patent office on 1998-06-23 for gan single crystal.
This patent grant is currently assigned to Mitsubishi Cable Industries, Ltd.. Invention is credited to Kazumasa Hiramatsu, Hiroaki Okagawa, Kazuyuki Tadatomo, Shinichi Watabe.
United States Patent |
5,770,887 |
Tadatomo , et al. |
June 23, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
Gan single crystal
Abstract
A GaN single crystal having a full width at half-maximum of the
double-crystal X-ray rocking curve of 5-250 sec and a thickness of
not less than 80 .mu.m, a method for producing the GaN single
crystal having superior quality and sufficient thickness permitting
its use as a substrate and a semiconductor light emitting element
having high luminance and high reliability, comprising, as a
substrate, the GaN single crystal having superior quality and/or
sufficient thickness permitting its use as a substrate.
Inventors: |
Tadatomo; Kazuyuki (Itami,
JP), Watabe; Shinichi (Itami, JP), Okagawa;
Hiroaki (Itami, JP), Hiramatsu; Kazumasa
(Yokkaichi, JP) |
Assignee: |
Mitsubishi Cable Industries,
Ltd. (Hyogo, JP)
|
Family
ID: |
27297960 |
Appl.
No.: |
08/320,263 |
Filed: |
October 11, 1994 |
Foreign Application Priority Data
|
|
|
|
|
Oct 8, 1993 [JP] |
|
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5-253098 |
Mar 31, 1994 [JP] |
|
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6-062813 |
Mar 31, 1994 [JP] |
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6-062815 |
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Current U.S.
Class: |
257/613; 257/103;
257/78; 257/76; 257/12; 257/614 |
Current CPC
Class: |
H01L
33/0075 (20130101); H01L 33/32 (20130101); C30B
29/406 (20130101); H01L 33/0093 (20200501); C30B
25/02 (20130101) |
Current International
Class: |
H01L
33/00 (20060101); C30B 25/02 (20060101); C30B
035/00 (); H01L 033/00 (); H01L 029/14 () |
Field of
Search: |
;257/613,78,77,103,94,12,76,614 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Nakamura "In Situ Monitoring of GaN Growth using Interference
Effects" Japanesse Journal of Applied Physics vol. 30, No. 8, pp.
1620-1627, Aug. 1991. .
Nakamura et al. "In Situ Monitoring and Hall Measurements of GaN
Grown with GaN Buffer Layers" Journal of Applied Physics, vol. 71,
No. 7, Jun. 1992. .
Detchprohm et al. "Hydride vapor Phase Epitaxial growth of a High
Quality GaN Film using a ZnO Buffer Layer" Applied Physic Letters,
vol. 6, No. 22, Nov. 1992..
|
Primary Examiner: Thomas; Tom
Assistant Examiner: Williams; Alexander Oscar
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas, PLLC
Claims
What is claimed is:
1. A GaN single crystal having a full width at half-maximum of the
double-crystal X-ray rocking curve of 5-250 sec and a thickness of
not less than 80 .mu.m.
2. The GaN single crystal of claim 1, which is produced by the
steps of:
(A) growing on a substrate, a material having a good lattice
constant match with a GaN single crystal, wherein said substrate
has a first GaN single crystal at least on its surface, and wherein
the material comprises an oxide selected from the group consisting
of BeO, MgO, ZnO, SrO, CdO, BaO and HgO to form a buffer layer,
(B) growing a GaN single crystal on the buffer layer of (A) to give
a second GaN single crystal, and
(C) chemically removing the buffer layer to separate the second GaN
single crystal.
3. A semiconductor light emitting element comprising:
(A) a substrate comprising a GaN single crystal having a full width
at half-maximum of the double-crystal X-ray rocking curve of 5-250
sec wherein the thickness of the substrate is at least 80
.mu.m,
(B) plural GaN group compound semiconductor layers comprising a
light emitting part wherein said layers are laminated on the
substrate of (A) to give a laminate having two outermost layers,
and
(C) an electrode set on each of the two outermost layers of the
laminate.
4. The semiconductor light emitting element of claim 3, wherein the
GaN single crystal is obtained by growing a material showing good
lattice match with GaN single crystal, on a substrate having GaN
single crystal at least on its surface, to give a buffer layer,
growing GaN crystal thereon to give a GaN single crystal, the above
steps being one cycle of crystal growth, and repeating said cycle
of crystal growth at least once on the GaN single crystal obtained
by the first cycle of crystal growth.
5. The semiconductor light emitting element of claim 3, wherein the
light emitting part has a p-n junction selected from the group
consisting of a homo-junction, a single-hetero junction, a
double-hetero junction, a single quantum well structure and a
multiple quantum well structure.
6. The GaN single crystal of claim 1, which is produced by the
steps of:
(A) growing on a substrate, a material having a good lattice
constant match with a GaN single crystal, wherein said substrate
has a first GaN single crystal at least on its surface to form a
buffer layer,
(B) growing GaN crystal on the buffer layer of (A) to give a second
GaN single crystal, the above steps (A) and (B) being a single
cycle of crystal growth,
(C) repeating said single cycle of crystal growth at least once on
the GaN single crystal obtained in step (B), and,
(D) removing the buffer layer by chemically separating the GaN
single crystals.
7. A semiconductor light emitting element of claim 3 which is
produced by the steps of:
(A) growing on a GaN single crystal substrate, a material having a
good lattice constant match with a GaN single crystal, wherein said
substrate has a first GaN single crystal at least on its surface,
to form a buffer layer,
(B) growing a GaN single crystal on the buffer layer of (A) to give
a second GaN single crystal, and
(C) chemically removing the buffer layer to separate the second GaN
single crystal.
8. A semiconductor light emitting element of claim 3, which is
produced by:
(A) growing on a GaN single crystal substrate, a material having a
good lattice constant match with a GaN single crystal, wherein said
substrate has a first GaN single crystal at least on its surface to
form a buffer layer,
(B) growing GaN crystal on the buffer layer of (A) to give a second
GaN single crystal, the above steps (A) and (B) being a single
cycle of crystal growth,
(C) repeating said single cycle of crystal growth at least once on
the GaN single crystal obtained in step (B), and,
(D) removing the buffer layer by chemically separating the GaN
single crystals.
Description
FIELD OF THE INVENTION
The present invention relates to a GaN single crystal which is
sufficiently thick and superior in quality, production thereof and
to a semiconductor light emitting element comprising said GaN
single crystal as a crystal substrate. The GaN single crystal can
serve well as a GaN single crystal substrate for a semiconductor
light emitting element comprising a light emitting part composed of
a poly-compound semiconductor particularly having GaN as one
component thereof.
In the description to follow, the crystal substrate is simply
referred to as substrate. In addition, compound semiconductors
having GaN as one component thereof, such as a binary mixed crystal
GaN, poly mixed crystals GaAlN, GaBN, InGaAlN, InGaAlBN etc. are to
be referred to as GaN group compound semiconductors.
BACKGROUND OF THE INVENTION
Motivated by a demand for multicolor light emitting displays and
improved data density in communication and recording, there is a
strong desire for a semiconductor light emitting element capable of
emitting light of shorter wavelength ranging from a blue light
wavelength to an ultraviolet wavelength.
GaN is drawing much attention as a semiconductor material to be
used for said semiconductor light emitting element. The direct
transition type band structure of GaN permits highly efficient
emission of light. GaN, moreover, emits light of shorter wavelength
ranging from a blue light wavelength to an ultraviolet wavelength,
due to a great band gap at room temperature of about 3.4 eV, thus
rendering itself suitable for the above-mentioned semiconductor
devices.
On the other hand, GaN requires a high crystal growth temperature
(a temperature where crystal growth can take place), at which
temperature the equilibrium vapor pressure of nitrogen is high. The
high pressure makes production of a bulky single crystal of good
quality from a molten solution of GaN very difficult. Accordingly,
a GaN single crystal has been heteroepitaxially formed on a
sapphire substrate or SiC substrate superior in heat resistance by
MOVPE (Metal Organic Vapor Phase Epitaxy) or MBE (Molecular Beam
Epitaxy).
In recent years, however, there has been reported a method (see
Applied physics letter Vol.61 (1992) p.2688) wherein ZnO is formed
on a sapphire substrate as a buffer layer and GaN single crystal is
formed on the ZnO buffer layer. This method has been conducive to
the improvement in the quality of GaN single crystal layer, as
compared with direct crystal growth on a sapphire substrate.
The aforementioned conventional method using ZnO as a buffer layer,
however, fails to produce a single crystal ZnO buffer layer of good
quality, since ZnO is formed on a sapphire substrate by sputtering.
The poor quality of the ZnO single crystal buffer layer affects the
quality of the GaN single crystal layer to be formed thereon, to
result in deficient crystal structure and presence of impurities in
the GaN single crystal layer obtained, thus failing to produce a
GaN single crystal layer of superior quality.
Even the conventionally known GaN single crystal of the highest
quality has a full width at half-maximum of the double-crystal
X-ray rocking curve of about 100 sec and mobility at room
temperature of 600 cm.sup.2 /VS. Due to the MOVPE done for growing
a crystal layer, however, the layer is obtained only at a thickness
of about 5 .mu.m at most and it is difficult to separate the GaN
single crystal from the substrate on which it has been formed and
use same, for example, as a substrate for a semiconductor light
emitting element. For this reason, GaN single crystals have been
used along with the substrates on which they are formed.
In the following description, the full width at half-maximum of the
double-crystal X-ray rocking curve is to be referred to as full
width at half-maximum, since, as used herein, it always denotes the
value of the double-crystal X-ray rocking curve. The full width at
half-maximum is to be abbreviated as FWHM. Accordingly, the FWHM
means full width at half-maximum of the double-crystal X-ray
rocking curve.
The GaN single crystal formed on a ZnO buffer layer by HVPE
(Hydride Vapor Phase Epitaxy) as mentioned above has a sufficient
thickness for a substrate, whereas its quality is poor as evidenced
by its FWHM which is not less than 300 sec.
In other words, there has never existed a GaN single crystal having
both good quality and sufficient thickness.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a GaN
single crystal having superior quality and sufficient thickness
permitting its use as a substrate.
Another object of the present invention is to provide a method for
producing the GaN single crystal having superior quality and
sufficient thickness permitting its use as a substrate.
A still another object of the present invention is to provide a
semiconductor light emitting element having high luminance and high
reliability, comprising, as a substrate, the GaN single crystal
having superior quality and sufficient thickness permitting its use
as a substrate.
The GaN single crystal of the present invention has high quality to
the extent that FWHM thereof is 5-250 sec and is sufficiently thick
(not less than 80 .mu.m) to permit its use as a substrate.
The above-mentioned GaN single crystal having superior quality and
sufficient thickness can be produced by the method of the present
invention. To be specific, a material showing good lattice match
with GaN single crystal is grown on a first substrate having GaN
single crystal at least on its surface; using the formed layer as a
buffer layer, GaN crystal is grown thereon to give a GaN single
crystal layer of hither quality; using the formed GaN single
crystal layer as a new substrate, a buffer layer is grown thereon;
and a GaN single crystal layer is formed on the buffer layer. For
short, the method of the present invention is alternative epitaxial
growth of buffer layer and GaN single crystal layer wherein the
repetitive growth affords high quality of GaN single crystal.
The GaN single crystal thus obtained has good quality and can have
a thickness of not less than 80 .mu.m on demand to be suitably used
as a substrate for a semiconductor light emitting element.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically shows one embodiment of the steps for
producing GaN single crystal by the method of the present
invention.
FIG. 2 schematically shows a three-layer structure of a
substrate.
FIG. 3 schematically shows one embodiment of the structure of a
light emitting element wherein the GaN single crystal of the
present invention is used as a substrate.
FIG. 4 schematically shows another embodiment of the structure of a
light emitting element wherein the GaN single crystal of the
present invention is used as a substrate.
FIG. 5 schematically shows a still another embodiment of the
structure of a light emitting element wherein the GaN single
crystal of the present invention is used as a substrate.
FIG. 6 schematically shows one embodiment of the structure of a
conventional light emitting element wherein sapphire crystal is
used as a substrate.
DETAILED DESCRIPTION OF THE INVENTION
The GaN single crystal of the present invention has a full width at
half-maximum of the double-crystal X-ray rocking curve of 5-250 sec
and has a thickness of not less than 80 .mu.m. Accordingly, the
single crystal of the present invention is superior in quality and
thick enough to be used as a substrate, which properties being
never concurrently achieved by conventional GaN single
crystals.
Such GaN single crystal can be produced, for example, by the method
of the present invention which is to be described in the
following.
The method for producing the GaN single crystal of the present
invention is most simply shown by step 1 in FIG. 1.
That is, a substrate having GaN single crystal at least on its
surface is used as a first substrate P.sub.0. On the first
substrate P.sub.0 is precipitated a material showing good lattice
match with the GaN single crystal, to give a buffer layer B.sub.1.
Then, a GaN single crystal P.sub.1 is formed on the buffer layer
B.sub.1.
This GaN single crystal P.sub.1 is the production target. Said GaN
single crystal has improved crystal quality as compared with the
GaN single crystal of the first substrate P.sub.0.
Also, the method for producing the GaN single crystal of the
present invention is shown by step 2 in FIG. 1.
In this step, the laminate (P.sub.0 +B.sub.1 +P.sub.1) obtained in
step 1 is used as a new substrate, on which a material having good
lattice match with GaN single crystal is grown to give a buffer
layer B.sub.2. A GaN single crystal P.sub.2 is grown on the buffer
layer B.sub.2. The step including forming a buffer layer on the GaN
single crystal obtained in the previous step as a substrate and
growing a GaN single crystal on the buffer layer is one cycle of
repetitive unit in the present invention.
Then, counting step 1 as a first cycle of the crystal growth, the
repetitive step is performed n times to give a laminate having a
GaN single crystal P.sub.n at the uppermost layer. The buffer
layers accumulated so far in the obtained laminate are removed to
give plural GaN single crystals P.sub.1 to P.sub.n.
In this method, the more the number of repeats, the better the
quality of the GaN crystal. However, the quality of the crystal
reaches an equilibrium state at a certain point. After the crystal
quality has reached the equilibrium state, the subsequent
repetition of the cycle is useful for producing a large number of
GaN single crystal substrates, rather than for improving the
crystal quality.
Further, the method for producing the GaN single crystal of the
present invention is shown by step 3 in FIG. 1.
In this step, the buffer layer B.sub.1 of step 1 is removed to
separate the GaN single crystal P.sub.1. Using P.sub.1 as a new
substrate, a material having good lattice match with GaN single
crystal is grown to give a buffer layer B.sub.2. A GaN single
crystal P.sub.2 is formed on the buffer layer B.sub.2 and the
buffer layer B.sub.2 is removed to separate the GaN single crystal
P.sub.2. As described, one cycle of the repetitive unit includes
forming of a buffer layer on the GaN single crystal obtained in the
previous step as a substrate, growing a GaN single crystal thereon
and removing the buffer layer to separate the GaN single crystal
thus obtained. Thus, n GaN single crystals P.sub.1 to P.sub.n can
be obtained by repeating the above-mentioned repetitive unit n
times, counting step 1 as a first cycle of the crystal growth.
Alternatively, the present method includes removing the buffer
layer on which a new GaN single crystal has been grown, at the end
of every cycle and separating the newly-formed GaN single crystal
into an independent layer. The improvement in quality in this step
is as described in step 2 above. In addition, the two GaN single
crystals obtained by removing the buffer layer at every cycle of
repetitive unit may be used as materials for other products or as
the substrates for the next GaN crystal growth.
A combination of the steps 2 and 3 as appropriate may be also used.
That is, the buffer layers are removed all at once upon repeats of
the repetitive unit for the desired number of times k. The optional
number k may be selected as desired.
The total number of the cycles of repetitive unit of the
aforementioned crystal growth is subject to no particular
limitation and it may be selected according to the desired quality
of GaN single crystal or the number of GaN single crystals needed.
For use as a substrate for conventional semiconductor devices, GaN
single crystal requires 2 to 5 times of repetition of the
aforementioned cycle.
For forming a buffer layer on a substrate, known methods such as
sputtering and CVD, or various epitaxial methods are used.
Sputtering is preferable in that layers are easily formed. A method
permitting epitaxial growth is particularly preferable for
improving the quality of the GaN single crystal obtained.
A method for growing a GaN single crystal on a buffer layer is
preferably that permitting epitaxial growth in view of the
improvement in the quality of crystal.
Examples of the method permitting epitaxial growth of the materials
to form GaN single crystal and buffer layer include VPE (Vapor
Phase Epitaxy), HVPE, MOVPE, MBE, GS-MBE (Gas Source MBE) and CBE
(Chemical Beam Epitaxy).
When a buffer layer and a GaN single crystal thereon are formed by
the same epitaxial growth method, consecutive growth in situ from
the buffer layer on to the GaN single crystal can be performed only
by changing the material to be fed.
While the buffer layer may be removed by any method insofar as it
can separate the GaN single crystal obtained, chemical removal with
an acid etc. is effective.
The first substrate P.sub.0 has GaN single crystal at least on the
surface thereof. The substrate may be that substantially made of
GaN alone in its entirety or that having GaN single crystal only on
the surface thereof.
When the latter is used, the material to become the substrate for
growing the GaN single crystal on the surface preferably has
superior resistance to the heat (1000.degree.-1100.degree. C.)
necessary for the growth of GaN single crystal and is exemplified
by sapphire crystal substrate, Si substrate, rock crystal, ZnO
substrate and SiC substrate.
Of the substrates having GaN single crystal only on the surface as
described above, the following is more preferable as the first
substrate P.sub.0.
The desirable substrate has a three-layer structure comprising
sapphire crystal as a substrate 1, a buffer layer 2 of Ga.sub.j
Al.sub.1-j N (wherein 0.ltoreq.j.ltoreq.1) formed thereon, and a
surface layer 3 of GaN single crystal to be formed on the buffer
layer. The three-layer substrate is preferable as a first substrate
P.sub.0, with the surface thereof being GaN single crystal of
superior quality. The technique described in Journal of Applied
Physics Vol.71 (1992) p.5543 and Japanese Journal of Applied
Physics Vol.30 (1992) p.1620 may be applied to the production of
the three-layer substrate.
The composition ratio j in Ga.sub.j Al.sub.1-j N of the buffer
layer 2 is not limited to a certain number with respect to GaN
single crystal and may vary depending on production conditions such
as growth temperature, growth pressure, feeding speed of the
starting material etc.
While the thickness of the buffer layer 2 is not limited, it is
preferably about 50 .ANG. to 1000 .ANG., at which thickness the
crystallinity of the GaN single crystal to grow on the buffer layer
becomes most desirable.
While the thickness of the surface layer 3 of GaN single crystal is
not limited, it is preferably not less than 0.3 .mu.m, with which
thickness the first substrate P.sub.0 becomes most preferable.
The Ga.sub.j Al.sub.1-j N buffer layer 2 to be formed on the
sapphire crystal substrate 1 and the surface GaN single crystal
layer 3 to be formed on the buffer layer 2 are preferably formed by
epitaxial growth as described in the above.
The material for the buffer layer to be used for producing the GaN
single crystal of the present invention is preferably a material
having a lattice constant that matches well with that of GaN single
crystal. To be specific, a preferable material has a crystal
structure of the wurtzite type and a lattice constant at the a-axis
of crystal lattice that matches preferably within .+-.10%, more
preferably within .+-.5% with the lattice constant of GaN single
crystal.
Examples of such material are compounds obtained by the use of one
or more members from the oxides of the group II elements. Examples
of the oxides of the group II elements include BeO, MgO, CaO, ZnO,
SrO, CdO, BaO and HgO.
Of the oxides of the group II elements, the more preferable
materials for the buffer layer are HgO, BeO and ZnO having the
wurtzite type crystal structure as does GaN single crystal.
In particular, ZnO has a lattice constant at the a-axis of 3.2496
.ANG., which is +1.9% relative to 3.189 .ANG. of the lattice
constant at the a-axis of GaN and is very close, and promises good
crystal growth of GaN.
In addition, ZnO permits good removal by etching with an acid and
is suitable as a material for a buffer layer.
In the present invention, moreover, a compound of the following
formula (I), made of BeO, ZnO and HgO is a suitable material for a
buffer layer to form GaN single crystal thereon.
wherein 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1 and
0.ltoreq.x+y.ltoreq.1.
Of the compounds of the formula (I), (BeO).sub.x (ZnO).sub.1-x is
particularly suitable for a buffer layer to grow GaN single crystal
thereon.
The composition ratios x and y in the above formula are selected in
such a manner that the lattice constant of the material designated
by the above formula (I) is similar to or the same as the lattice
constant of GaN single crystal under various production
conditions.
In the present invention, moreover, oxides of the group II elements
other than the aforementioned BeO, ZnO and HgO, such as MgO, CaO,
SrO and CdO are useful as the material for the buffer layer.
While the crystal structure of these compounds is not of wurtzite
type but of zinc blend type, when added to a compound of the
formula (I), which is of wurtzite type, in a small amount, the
overall crystal structure can be maintained wurtzite type and the
obtained compound can be used as a preferable material for the
buffer layer. Accordingly, one or more members from MgO, CaO, SrO,
CdO and BaO is(are) added to a compound of the formula (I) as the
central skeleton in an amount where the wurtzite type crystal
structure is maintained, or added thereto to substitute the
compound to give a material compound for the buffer layer.
Examples of such material compound include (MgO).sub.k (BeO).sub.m
(ZnO).sub.n (HgO).sub.1-k-m-n, (Mgo).sub.k (BeO).sub.m
(ZnO).sub.1-k-n and (Mgo).sub.k (ZnO).sub.1-k. In particular,
(MgO).sub.k (ZnO).sub.1-k is a desirable material for the buffer
layer for forming GaN single crystal.
The composition ratios k, m and n in the above formulas are
selected in such a manner that the lattice constant of the compound
is similar to or the same as the lattice constant of GaN single
crystal under various production conditions, as in the case of the
compound of the formula (I).
While the thickness of the buffer layer is not limited, it is
generally 0.01 .mu.m-2 .mu.m, preferably 0.02 .mu.m-2 .mu.m, and
most preferably 0.02 .mu.m-1.5 .mu.m. With this range of thickness,
the crystallinity of the desired single crystal to be formed on the
buffer layer becomes fine.
As described above, the method for producing the GaN single crystal
of the present invention improves the quality of the GaN single
crystal every time the cycle of crystal growth is repeated.
Many dislocations and deficiencies present in the GaN single
crystal substrate can be reduced in the interface between the
substrate and the buffer layer, within the buffer layer, in the
interface between the buffer layer and GaN single crystal and
within the GaN single crystal. A multitude of repeats of the cycle
is considered to gradually bring the quality of the GaN single
crystal to the level set depending on the growth conditions
etc.
The quality of the GaN single crystal can be still more improved by
selecting the first substrate and the material of the buffer layer
as stated supra.
The method of the present invention affords a single crystal having
superior quality and a crystal structure exhibiting an FWHM of
5-250 sec, reduces dislocations and deficiencies conventionally
found during the growth, and provides a crystal having superior
quality and sufficient thickness, in the absence of dislocations
even when the crystal growth is done for an extended period of
time.
The quality of the GaN single crystal of the present invention is
expressed numerically using the value of full width at half maximum
obtained by X-ray diffraction. The X-ray diffraction is based on
the diffraction of the X-ray irradiated on crystal. In the present
invention, measurement was done with the use of double-crystal, so
as to improve measurement precision.
The X-ray diffraction using double-crystal permits evaluation of
lattice constant of a sample crystal with precision and evaluation
of the completeness of the crystal based on half-value width.
For evaluation of the quality of the GaN single crystal of the
present invention, the X-ray entering from an X-ray source was
monochromized to a high degree by the first crystal and irradiated
on the GaN single crystal (second crystal) of a sample, whereafter
full width at half-maximum about the peak of the X-ray diffracted
from the sample was measured.
As the X-ray source, Cuk .alpha..sub.1 was used and X-ray was
generated at 30 kV and 10 m.ANG.. For monochromization, Ge (400)
was used as the first crystal. The measurement was done with
respect to the diffraction peak of GaN (0002) at a step interval of
measurement of 0.002.degree..
A GaN single crystal was produced by the method of the present
invention and the quality of the single crystal was evaluated, the
results of which follow hereunder.
EXPERIMENTAL EXAMPLE 1
As the first substrate P.sub.0 to be used in the production of the
GaN single crystal of the present invention, a substrate wherein
GaN single crystal had been epitaxially grown on a sapphire crystal
substrate by MOVPE was used. The buffer layer had a thickness of
0.2 .mu.m and was made of ZnO. The cycle of crystal growth was
repeated 5 times. The thickness of the GaN single crystals P.sub.1
to P.sub.5 formed at every crystal growth cycle was 300 .mu.m. For
repeating the cycle of crystal growth, a buffer layer and a GaN
single crystal were sequentially grown on the first substrate
P.sub.0 to give a laminate and all buffer layers were removed at
once to separate the GaN single crystals P.sub.1 to P.sub.5, as
shown in step 2 in FIG. 1.
The GaN single crystal P.sub.5 obtained last had an FWHM of 29 sec.
The thickness of the GaN single crystal was 305 .mu.m.
EXPERIMENTAL EXAMPLE 2
GaN single crystals were prepared in the same manner as in
Experimental Example 1 except that the buffer layer formed
previously was removed upon each epitaxial growth of a GaN single
crystal and the obtained GaN single crystal was used as a new
substrate for the next cycle, as shown in step 3 in FIG. 1, instead
of repeating the cycle of crystal growth as in step 2.
The GaN single crystal P.sub.5 obtained last had an FWHM of 28 sec.
The thickness of the GaN single crystal was 289 .mu.m.
EXPERIMENTAL EXAMPLE 3
GaN single crystals were prepared in the same manner as in
Experimental Example 2 except that a substrate having a three-layer
structure composed of a sapphire substrate, an AlN (i.e.
composition ratio x in Ga.sub.x Al.sub.1-x N being 0) buffer layer
and a GaN single crystal was used as the first substrate.
Referring to FIG. 2, production of the three-layer substrate is
explained briefly in the following. As a buffer layer 2, AlN (i.e.
composition ratio x in Ga.sub.x Al.sub.1-x N being 0) was
epitaxially grown to the thickness of 500 .ANG. on a sapphire
crystal substrate 1 (thickness 300 .mu.m, 5 cm.times.5 cm) by
MOVPE; the material gas, alone, was changed and a GaN single
crystal was epitaxially grown to the thickness of 2 .mu.m by the
same MOVPE to give a surface layer 3; whereby a three-layer
substrate of the sapphire crystal substrate 1, the AlN buffer layer
2 and the GaN single crystal surface layer 3, total thickness being
about 302 .mu.m, was prepared.
The GaN single crystal P.sub.5 obtained last had an FWHM of 25 sec.
The thickness of the GaN single crystal was 295 .mu.m.
EXPERIMENTAL EXAMPLE 4
GaN single crystals were prepared in the same manner as in
Experimental Example 2 except that a three-layer substrate as used
in Experimental Example 3 was used as the first substrate P.sub.0
and (BeO).sub.0.13 (ZnO).sub.0.87 was used as the material for the
buffer layer in each cycle of crystal growth of GaN single
crystal.
The GaN single crystal P.sub.5 obtained last had an FWHM of 28 sec.
The thickness of the GaN single crystal was 301 .mu.m.
EXPERIMENTAL EXAMPLE 5
With the aim of examining the quality of a conventional GaN single
crystal, a 0.6 .mu.m-thick buffer layer of ZnO was formed on a
sapphire crystal substrate (thickness 300 .mu.m, 5 cm.times.5 cm)
by sputtering and a GaN single crystal was epitaxially grown
thereon to the thickness of 250 .mu.m by HVPE.
The GaN single crystal P.sub.5 obtained last had an FWHM of 420
sec.
As is evident from the above-mentioned experiment results, the
method for producing the GaN single crystal of the present
invention enables production of a GaN single crystal of superior
quality that has never been produced by conventional methods.
The quality of the GaN single crystal can be still more improved by
the use of the aforementioned three-layer substrate as the first
substrate and selecting the material of the buffer layer as
mentioned above.
It is also an advantageous effect of the present invention that a
GaN single crystal sufficiently thick for use as a substrate can be
obtained.
The thick GaN single crystal of superior quality which is produced
by the method of the present invention can be suitably used for
semiconductor light emitting elements such as light emitting diode
(LED), laser diode (LD) and superluminescence diode, and electron
devices. In the electron devices, the use of the GaN single crystal
of the present invention as a substrate enables production of LED,
LD etc. having the same electrode structure as in the conventional
red LED etc. Those which emit blue lights are particularly
important. In addition, the efficiency of the light emission of
semiconductor light emitting elements by the use of the GaN single
crystal of the present invention is advantageously high.
Examples of the semiconductor light emitting element wherein the
GaN single crystal of the present invention is used as a substrate
are shown in the following.
FIG. 3 schematically shows the structure of LED of a typical
semiconductor light emitting element. As shown in the Figure, the
LED comprises a laminate A (4, 5, 6) including the GaN single
crystal (n type) produced by the method of the present invention as
a substrate 4, and a semiconductor layer 5 (n type) and a
semiconductor layer 6 (p type), both being GaN group compound
semiconductors, formed thereon, and electrodes 8 and 7 set on the
outermost layers 6 and 4 of the laminate A.
By using such GaN single crystal of superior quality as a
substrate, a light emitting element can be formed between a pair of
electrodes facing each other, which is capable of highly efficient
light emission with greater reliability.
A light emitting element by the use of a conventional GaN type
single crystal is formed on a sapphire substrate S, as shown in
FIG. 6. However, electrodes cannot be formed on the substrate,
since the sapphire substrate is an insulating body. For this
reason, the positive and negative electrodes 7c and 8 are formed on
the upper side of the layer constituting the light emitting part in
such a manner that the both electrodes face the sapphire substrate
S. Such structure poses problems in that the LED cannot be designed
or handled during production and mounting as are conventional LEDs,
and that the light emitting area is small. In addition, a
conventional GaN single crystal formed on a sapphire substrate is
insufficient in crystal quality and light emitting efficiency, due
to a great mismatching between the lattice constants of the
sapphire and the GaN.
The conductive types p and n of the semiconductor layers 4, 5 and 6
in FIG. 3 may be otherwise. The combination of the components of
the GaN group compound semiconductor forming the light emitting
part may be any insofar as it permits p-n junction and emits light
upon application of forward current. For example, GaNs which are
homoepitaxially grown on GaN single crystal substrates and GaN
group compound semiconductors of the formula In.sub.1-r (Ga.sub.1-t
Al.sub.t).sub.r N (wherein t and r is 0-1) heteroepitaxially grown
on GaN single crystal substrates are preferably used.
In FIG. 3, the light emitting part has a simple two-layer p-n
junction. The junction of the light emitting part may be
homo-junction where the same materials are joined, or
hetero-junction where different materials are joined. Furthermore,
the junctional structure of the light emitting part is not limited
to two-layer junction but may be multi-layer junction such as
double-hetero junction, single quantum well, multiple quantum well
etc.
With such junctional structure of the light emitting part, various
semiconductor light emitting elements such as LED and LD are
obtained.
The electrode to be formed on a substrate may be as 7a shown in
FIG. 3, wherein it is formed on the outermost surface of the
substrate, or 7b shown in FIG. 5, wherein it is formed on the side
of the substrate.
LEDs were manufactured using the GaN single crystal obtained by the
method of the present invention as a substrate and the quality
thereof was examined.
The LED had a double hetero junction structure wherein an n-AlGaN
cladding layer 5, an undoped InGaN active layer 9, and a p-AlGaN
cladding layer 6 are sequentially grown on a GaN single crystal
substrate 4 obtained by the method of the present invention (see
FIG. 4). The substrate 4 had an FWHM of 30 sec, 100 sec or 250 sec.
The thickness was all 280 .mu.m. The composition ratio of InGaN of
the active layer was In.sub.0.15 Ga.sub.0.85 N or In.sub.0.25
Ga.sub.0.75 N. A light emitting element was produced with respect
to InGaN of respective composition ratio and subjected to
experiment.
Alongside therewith, LEDs comprising a conventional GaN single
crystal or a sapphire crystal as a substrate were prepared and
compared with the LEDs wherein the GaN single crystal of the
present invention was used as a substrate in terms of quality. The
FWHM of the conventional GaN single crystal was 300 sec.
The LEDs were evaluated as to the initial luminance and service
life. The service life was evaluated according to three levels of
percent decrease in luminance (A: decrease percentage of less than
2%, B: decrease percentage of from 2 to less than 5%, C: decrease
percentage of from 5 to 10%) calculated in relative proportion of
the luminance upon 2000 hours of continuous light emission by the
application of 20 m.ANG. current at 85.degree. C. in 85% humidity,
to the initial luminance. The results are shown in the following
Tables 1 and 2.
TABLE 1 ______________________________________ Luminance and life
of LED with active layer of In.sub.0.15 Ga.sub.0.85 N FWHM Initial
luminance Substrate (sec) (candela) Life
______________________________________ GaN 30 1.8 A GaN 100 1.4 A
GaN 250 1.2 B GaN 300 1.1 B Sapphire -- 1.0 C
______________________________________
TABLE 2 ______________________________________ Luminance and life
of LED with active layer of In.sub.0.25 Ga.sub.0.75 N FWHM Initial
luminance Substrate (sec) (candela) Life
______________________________________ GaN 30 2.9 A GaN 100 2.5 A
GaN 250 2.2 B GaN 300 2.2 B Sapphire -- 2.0 C
______________________________________
As is evident from Tables 1 and 2, the LEDs wherein the GaN single
crystal with superior quality of the present invention was used as
a substrate were generally superior to the conventional LEDs in
terms of initial luminance and service life.
With regard to LD, the following phenomenon was confirmed.
In a conventional LD wherein a sapphire crystal is used as a
substrate, the substrate surface does not form a desirable mirror
state, since the sapphire crystal hardly permits easy formation of
cleavage plane. Accordingly, the surface of the GaN group compound
semiconductor layer to be formed on the substrate is affected by
the surface condition of the substrate, so that a desirable
reflecting surface for LD cannot be formed. In contrast, the GaN
single crystal of the present invention has high quality and
sufficient thickness, which enables feasible formation of cleavage
plane on the GaN single crystal substrate.
An LD wherein a conventional GaN group compound semiconductor was
used failed in stimulated emission by current injection, due to
inferior quality of the crystal. Then, a stripe laser of
Fabry-Pe.GAMMA.rot resonator with the use of the GaN single crystal
of the present invention as a substrate was constructed and
subjected to experiment. As a result, the stimulated emission
occurred at room temperature.
* * * * *